The present invention relates to the improvement of a battery pack incorporating a PTC device therein. In particular, the present invention relates to a battery pack and a PTC device incorporated in such battery pack wherein a plurality of unit cells placed side by side in parallel are connected in series with the PTC device.
PTC devices are incorporated in battery packs as protection devices. The PTC device connected in series with unit cells will, when an overcurrent flows through the cells or a temperature of the cells rises, protect the cells by increasing its electrical resistance rapidly so that the cell current is shut off or reduced to a virtually zero value.
A battery pack having a PTC device connected to unit cells in series is described for example in Japanese Patent Laid-open Publication (Tokkai Hei) No. 9-63553. As shown in FIG. 1, the battery pack described in this publication has the PTC device P connected between two unit cells 1 which are positioned linearly. The PTC device P has a top surface connected to an end electrode 1A of one unit cell 1 and a bottom surface connected to an end electrode 1A of the other unit cell. This structure is suitable for incorporating the PTC device in the battery pack where the unit cells are positioned linearly. However, such a PTC device cannot connect cells in a desirable condition in a battery pack where the unit cells are placed side by side in parallel.
A battery pack incorporating a PTC device and having unit cells placed in parallel is described in Japanese Utility Model Laid-open Publications (Jikkai Hei) Nos. 2-69440 and 6-38157. FIG. 2 shows an exploded perspective view of the battery pack described in the former publication. FIGS. 3 and 4 show the PTC device incorporated in the battery pack and the battery pack incorporating the PTC device as described in the latter publication.
The battery pack shown in FIG. 2 incorporates a PTC device P having a first electrode 3 and a second electrode 4 connected to a bottom surface and a top surface respectively of a rectangular PTC layer 2. The first electrode 3 and the second electrode 4 protrude from the both sides of the PTC layer 2 and are connected to the end electrodes 1A of the unit cells 1. In this structure, the battery pack has the PTC layer 2 between the adjacent electrodes 1A of the cells 1 placed in parallel.
The PTC device P shown in FIG. 3 has a PTC layer 2 which is disk-shaped having an external size which is generally the same as the diameter of the unit cell and which is formed to allow the convex electrode of the cell to be inserted. The PTC layer 2 in this form can be placed around the convex electrode. A plate-shaped first electrode 3 is connected to the bottom surface of the PTC layer 2. In order to insert the convex electrode, the first electrode 3 has a convex-shaped center portion which is inserted into the center hole 2A of the PTC layer 2. A second. electrode 4 having a semi-spherical section along the PTC layer 2 and a protruding lead is connected to the top surface of the PTC layer 2. The first electrode 3 is connected to the convex electrode, and the second electrode 4 is, as can be seen in FIG. 4, connected to the end electrode 1A of the adjoining unit cell 1.
The PTC devices shown in FIGS. 2 and 3 are convenient for incorporating in battery packs which have a plurality of unit cells 1 placed in parallel. In particular, the PTC device shown in FIG. 3 is characterized in that it can be placed around the convex electrode so that it hardly protrudes from the unit cell.
However, the battery packs described in those publications are limited by the area of the PTC layer so that it is difficult to make it larger. In the PTC device P shown in FIG. 2, the first electrode 3 and the second electrode 4 cannot be connected to the end electrodes 1A of the unit cells 1 if the PTC layer 2 is made larger. This PTC device P cannot be made wider than the spacing between the end electrodes 1A of the adjoining unit cells 1.
Also, in the PTC device P shown in FIGS. 3 and 4, the outer size of the PTC layer 2 cannot be made greater than the thickness of the unit cell 1. If it is made greater, it would protrude from the cell 1 so that the size of the battery pack is locally increased. Further, the PTC layer 2 in this form has the center hole 2B, which decreases the substantive area of the PTC layer 2 that can be used.
Since the PTC device is used while connected in series with respect to the unit cells, it is important that the internal resistance of the PTC device is small under normal conditions. This is because the PTC device consumes power in proportion to the product of the internal resistance of the PTC layer and the load current squared. Further, in battery packs used for large current applications, the current through the PTC device also increases. The PTC device can be designed to withstand the large currents by increasing the area of the PTC layer. In order to decrease the internal resistance and increase the large current, it is important to make the area of the PTC layer in the PTC device larger. However, battery packs heretofore had a shortcoming in that it was difficult to achieve this.
The present invention has been developed for the purpose of solving this shortcoming. An important purpose of the present invention is to provide a battery pack and a PTC device incorporated in such battery pack which could be used safely under large current loads by increasing the area and decreasing the internal resistance of the PTC layer.
The battery pack according to claim 1 of the present invention has a plurality of unit cells 1 placed side by side in parallel and a PTC device P to protect the cells 1 from overcurrents. The PTC device P has a first electrode 3 and a second electrode 4 attached to both surfaces of the PTC layer 2. The PTC device P is connected in series to the unit cells 1 by connecting the first electrode 3 and the second electrode 4 to the end electrodes 1A of the two unit cells 1 whose end electrodes 1A are positioned on the same or almost the same plane. It is intended that the term xe2x80x9calmostxe2x80x9d is used to include the concept of xe2x80x9csubstantially the same planexe2x80x9d.
The outer shape of the PTC layer 2 is formed to cover almost the entirety of the end surfaces of the two cells 1 to which the first electrode 3 and the second electrode 4 are connected. Further, the PTC layer 2 has through holes 2B positioned correspondingly to positions of the end electrodes 1A of the cells 1.
The first electrode 3 and the second electrode 4 attached to both surfaces of the PTC layer 2 are formed into an outer shape that is almost the same as the outer shape of the PTC layer 2; and the first electrode 3 and the second electrode 4 are connected to the end electrodes 1A of the unit cells 1 through their portions which are located correspondingly to the through holes 2B of the PTC layer 2.
The PTC device according to claim 5 of the present invention protects the unit cells 1 from the overcurrents by connecting the first electrode 3 and the second electrode 4 which are attached to the both surfaces of the PTC layer 2 to the end electrodes 1A of a plurality of the cells 1 placed side by side in parallel.
The outer shape of the PTC layer 2 is formed to cover almost the entirety of the end surfaces of the two cells 1 to which the first electrode 3 and the second electrode 4 are connected. Further, the PTC layer has the through holes 2B positioned against the end electrodes 1A of the cells 1.
The first electrode 3 and the second electrode 4 connected to both surfaces of the PTC layer 2 are formed into almost the same outer shape as the outer shape of the PTC layer 2; and the first electrode 3 and the second electrode 4 are connected to the end electrodes 1A of the unit cells 1 at points where the through holes 2B of the PTC layer 2 are located.
In the battery pack according to claim 2 and the PTC device according to claim 6 of the present invention, the second electrode 4 connected to the top surface of the PTC layer 2 has a convex section 4A which is inserted into the through hole 2B. The second electrode 4 has the convex section 4A connected to the end electrode 1A of the unit cell 1. It is noted that the top and bottom directions in the present specification are determined based on the drawings.
In the battery pack according to claim 3 and the PTC device according to claim 7 of the present invention, the first electrode 3 connected to the bottom surface of the PTC layer 2 has a convex section 3A protruding from the bottom surface at the position of the through hole 2B of the PTC layer 2. The convex section 3A is connected to the end electrode 1A of the unit cell 1.
The battery pack according to claim 4 of the present invention incorporates cylindrical cells as the unit cells 1, and the outer shapes of the PTC layer 2, the first electrode 3 and the second electrode 4 are rectangularly shaped with the corners bevelled to conform with the cylindrical cells.